CX25870/871
1.0 Functional Description
Flicker-Free Video Encoder with Ultrascale Technology
1.3 Device Description
Figure 1-10. Windows Desktop Image From CX25870 With Overscan Compensation
13.78 % / 2 =
6.89 % Blanking on Each Side
of TV
NOTE(S):
Overscan percentages taken from CX25870's Autoconfiguration Mode 0.
100381_073
In Figure 1-10, the CX25870 overscan compensated the 640 horizontal active
pixels of data to fit within the viewable video region. With 13.78 percent HOC,
the active data is contained within a 45.397 µs. portion of time within each active
line while the remaining 7.26 µs (52.65556 µs.–45.397 µs.) part of the active
region is blanked by the encoder.
The net result of overscan compensation will be an interlaced NTSC, PAL, or
SECAM video image that fits within the bezel area of a TV Monitor. Correct
choice of the HOC and Vertical Overscan Compensation (VOC) percentages is
important so that no regions of the active input image will be hidden behind the
plastic of the TV unit. Various TVs require different HOC and VOC values to
fully utilize the entire viewable area of the TV. For the user’s convenience,
Conexant has generated Appendix A in the CX25870/871 datasheet which lists
many of the possible overscan ratios for the 3 major desktop resolutions
(640x480, 800x600, and 1024x768) and the 2 most popular video outputs (NTSC
and PAL-BDGHI). Varying amounts of blanking would be required depending on
the HOC and VOC percentages and active input resolutions.
Ultimately, the blanked regions would be dictated by the BLANK* signal
itself and/or the internal pixel counter for the CX25870/871. Actual transmission
of null or blanked pixels is not necessary since the encoder ignores any data sent
to it via the pixel input port within the blanked regions. Only the active pixels
need to be sent to the encoder from the controller during the digital active period.
100381B
Conexant
1-35
CONEXANT [ CONEXANT SYSTEMS, INC ]
